There is an interest to be able to measure the height of targets observed by forward looking automotive radars. A reason is to reduce the risk of false automatic braking interventions due to elevated objects (e.g. fans in tunnel ceilings, over-head signs, and bridges) and objects at the road surface (e.g. manholes, tramway tracks, and cat's eyes). Hence the radar antenna's capability to estimate horizontal bearings to targets needs to be expanded to a capability of estimating both the horizontal and the vertical bearings. This estimating of both the horizontal and the vertical bearings of an object is known as “two-dimensional direction of arrival (2D DOA) estimation”.
EP 0 947 852 A1 provides a radar apparatus that detects a vertical bearing of an object in addition to a horizontal bearing of an object. The radar apparatus has a transmitting antenna comprising a transmitting element and a receiving antenna comprising an array of receiver elements and wherein one or more receiver elements is shifted with respect to the remaining receiver elements by the same vertical displacement. This vertical shift is used to detect a vertical bearing of the target object by using the phase mono-pulse method.
The inventers of the present disclosure have found the following.
There has been a need to provide a radar apparatus, in particular for automotive use, that can detect a vertical bearing of an elevated object in an improved manner, preferably without being influenced by multi-path fading due to the presence of the road surface.
The above problem could be solved if receiver elements at other vertical positions were added, thus adding vertical shifts to the receiving antenna. The signals from the true object and its reflection could be resolved. However this arrangement would result in a larger receiving antenna and also a more complex construction of the radar apparatus as a whole. In addition the vertical extent of each receiving element in the receiving antenna would have to be restricted to prevent the receiving element from overlapping neighboring elements above and below it.
EP 0987561 A2 discloses a radar device wherein two receiver elements are aligned with three transmitter elements which are switched on in sequence. The receiver elements are alternately connected to a signal processing portion in dependence of the switching of the transmitter elements. Therefore the radar device according to EP 0987561 A2 is able to acquire signals equivalent to ones obtained in radars having a single transmission antenna and six reception antennas, and to reduce the number of the reception antennas.